The present invention relates to a semiconductor device having a transistor with an increased current amplification factor, and more particularly, to a programmable read only memory (PROM) in which each memory cell with a vertical PNP transistor for preventing reverse current is improved in structure for decreasing the driving power consumption of a decoder/driver.
In general, a PROM includes a plurality of memory cells, and each memory cell consists of a vertical PNP transistor for preventing reverse current and a programmable element connected between the emitter of the vertical PNP transistor and a bit line. To write into or to read from the PROM, a word line and a bit line are selectively driven. The selective driving of a word line is carried out by a decoder/driver at a certain power consumption, and in order to increase the degree of integration and to lower the overall power consumption of the PROM, the driving power consumption of the decoder/driver for driving a word line should be kept as low as possible.
One way of achieving a lower driving power consumption of the decoder/driver is to increase the packing density of a memory cell size of the PROM, thus enabling each element to be driven with a small current. This approach is disclosed in an article by Toshitaka Fukushima, Kouji Ueno, and Kazuo Tanaka, entitled "A High Speed Schottky 4k-Bit PROM Using Diffused Eutectic Aluminum Process (Deap)", Proceedings of the 11th Conference (1979 International) on Solid State Devices, Tokyo, 1979; Japanese Journal of Applied Physics, Vol. 19 (1980) Supplement 19-1, pp175-180. In this reference, in order to suppress undesirable parasitic effects due to the increased packing density, two techniques, i.e., Shallow V Groove" (SVG) and Isolation by Oxide and Polysilicon (IOP) are employed.
In the conventional PROM, however, because the current amplification factor of the vertical PNP transistor for preventing reverse current is relatively small, the decoder/driver is still required to sink a relatively large current, as later described in detail. Therefore, there are problems in the conventional PROM wherein the decoder/driver occupies a large area and requires a high power consumption.